The invention relates to a bushing for the sealing passage of a cable, pipe and the like through a wall, comprising a frame made of rigid material which may be sealingly inserted in an aperture provided in the wall, as well as one of more parallelepipedal blocks of a resilient material such as rubber which may be composed from two identical halves, which blocks in their composite state exhibit a bore adapted with narrow tolerances to the shape and dimensions of a cable to be passed through, which blocks can be inserted in the frame aperture so as to be a fine fit, with the two halves of every block encasing a cable, whilst in additioning tightening means with a pressure plate are present which can compress the blocks inserted in the frame aperture in the plane of this aperture such that a sealing compression is effected between the two halves of any one block, between the outer faces of mutually adjacent blocks, between the wall of the bores and the outer surface of the cables contained therein, as well as between the outer faces of the blocks and the inner walls of the frame.
Such bushing is known from the European patent specification bearing the publication number 0 183 300.
Several drawbacks attach to such a known bushing, especially the following ones:
1. The blocks used in this known bushing are made of incompressible rubber, so that when tolerances of, say, more than 1 mm have occurred in the manufacture of the frame or the blocks, these blocks can hardly if at all be thrust into the frame, or they lie loose in the frame. PA1 2. The known bushing contains a rigid pressure plate with a thrust bolt as the principal tightening means. Threading the pressure bolt down causes a concentrated load--which in actual practice may amount to a few tons!--to be exerted upon the rigid pressure plate in order to achieve as uniform a distribution as practicable of downward directed forces acting upon the blocks inserted in the frame. By these forces all blocks in the frame should be deformed in such a way that a proper sealing of, for instance, cables passed through is ensured. PA1 In actual practice, however, the distribution of forces proves to be nowhere near uniform, because a counter-reaction of the non-compressible rubber mass of the blocks incited by the aforesaid enormous forces reduces the downward directed pressure especially near the sides of the frame. Also, in actual practice it is only the blocks disposed in the uppermost zone inside the frame which undergo the aforesaid requisite deformation and, in addition, these blocks absorb virtually the entire downward directed pressure. The said deformation develops outwards of the frame, which is obviously highly undesirable. PA1 The outcome of the drawbacks so far mentioned in item 2 proves to be in practice not only that the blocks with the cables passed through them in the uppermost zone of the frame are subject to severe deformation, which is generally found to be permanent, so that proper sealing has become impossible, but also that the blocks disposed in the bottom zone of the frame do not undergo the aforesaid requisite deformation, which again results in an inadequate sealing of, for instance, cables passed through, partly on account of the localized appearance of fissures between adjacent blocks. PA1 3. The pressure plate used in the known bushing is provided with a recess on its edges in order that this pressure plate may invariably be a true fit inside the frame even in the event of tolerances in the frame's dimensions. The said recess, however, causes an upward displacement of the rubber of the blocks which is undergoing deformation during compression. Under operating conditions of the bushing, an irregular edge of protruding rubber may appear on both edges of the pressure plate as a result. PA1 4. The known bushing is provided with rod-shaped elements which may be supported with their ends in two oppositely disposed grooves in flanges of the frame, whilst the central part of a rod-shaped element may be located in a groove of a half block such that it prevents any displacement in a transverse direction relative to the plane of the frame. PA1 As the rod-shaped elements are frequently of especially sturdy design in order to afford extra protection against displacement and the grooves are not generally given corresponding extra depth, it turns out in actual practice for reasons mentioned hereinbefore under item 2 that inadequate sealing is attained between rows of blocks which a rod-shaped element is confined. PA1 5. The frame used in the known bushing is composed by, for instance, welding angle irons or metal strips together. This construction gives rise to small continuous weld fissures or beads in the angles of the frame all its adverse consequences on the sealing. PA1 Owing to the non-compressibility of the rubber, the blocks cannot fill up these weld fissures in particular. PA1 6. In the known bushing, the downward directed forces produced by the tightening means should deform all blocks contained within the frame as well as, for instance, the resilient cables passed through in such a way that an effective sealing is obtained. This deformation frequently tends to lead to unacceptable damage to the cables, especially to cables passed through the uppermost zone of the frame. In addition, when armoured and/or non-compressible cables are passed through, only deformation of the blocks disposed within the frame takes place, leading to inadequate sealing. PA1 7. In actual practice the known bushing proves insufficiently fire-resistance, as will be elucidated hereinafter. PA1 A conflagration present on, for instance, one side of the known bushing under operating conditions reaches a temperature of about 900.degree. C. Heat from the conflagration is transmitted by PA1 From the heat-transfer coefficient of steel it becomes instantly clear that at the aforesaid conflagration temperature the side facing away from the conflagration will undergo a considerable rise in temperature (180.degree. C. is generally considered the maximum admissible). In consequence, the three first-named component parts will very soon show the admissible rise in temperature, if not protected by extra insulation. Depending upon the mass of the (copper) conductors, which in fact have an even higher coefficient of heat transfer than steel, a very rapid transmission of heat from the conflagration takes place. Adequate insulation of cables passed through is one mean of restricting a measurable rise in temperature on the side facing away from the fire. With the known bushing this is impossible, because there is no direct contact between the conductors and any heat-absorbing rubber. The rod-shaped elements and the rubber lead-through blocks will exhibit a very slight heat transfer (the K-value of heat-resistant rubber is about 0.18 W/mK). With aforesaid conflagration temperature, the inadequate fire resistance of the known bushing will cause cable sheathing to soften, to emit a good deal of smoke and even to ignite in an unfavourable situation. These effects are strengthened by the inadequate sealing provided by the known bushing, as has been expounded in the foregoing items 1 to 6 inclusive. Hot flue gases will be forced through all apertures by positive pressure in a burning space and ultimately ignite the flue gases emitted by insulating materials.
the frame PA2 through bolts PA2 the steel pressure plate PA2 conductors of cables passed through PA2 rod-shaped elements PA2 rubber lead-through blocks.